Gorilla Mux Router: Path Variable Extraction

In this tutorial, you'll learn about Gorilla Mux Router: Path Variable Extraction. We cover key concepts, practical examples, and best practices.

Gorilla mux variables -- Extract path variables from gorilla/mux routes correctly using mux.Vars() to access named parameters.

The Problem

gorilla/mux supports named path variables like /users/{id}, but they are not available from r.URL.Query(). You must use mux.Vars(r) to retrieve the map of path variables.

Wrong

r := mux.NewRouter()
r.HandleFunc("/users/{id}", userHandler)
func userHandler(w http.ResponseWriter, r *http.Request) {
    id := r.URL.Query().Get("id") // Empty!
}

Output:

$ curl http://localhost:8080/users/42
// id is empty string

Right

r := mux.NewRouter()
r.HandleFunc("/users/{id}", userHandler)
func userHandler(w http.ResponseWriter, r *http.Request) {
    vars := mux.Vars(r)
    id := vars["id"]
    w.Write([]byte("User: " + id))
}

Output:

$ curl http://localhost:8080/users/42
User: 42

Prevention

• Always import github.com/gorilla/mux and call mux.Vars(r)
• Named parameters use curly braces: {paramName}
• Use .Queries("key", "value") for query param constraints
• Use .Methods("GET"), .Host("...") for additional matching
• Access query params via r.URL.Query() alongside mux vars

Common Mistakes with router gorilla mux

1. Non-exhaustive pattern matches that compile with warnings then crash at runtime
2. Misunderstanding that String is [Char] with poor performance for large text operations
3. Using foldl instead of foldl' causing stack overflow on large lists

These mistakes appear frequently in real-world GO code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

FAQ

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